1. Field of the Invention
The present invention relates to a semiconductor device having an improved silicon oxide film as a gate insulation film of a Metal Insulator Semiconductor structure and a method of making the same.
2. Discussion of the Background
With the miniaturization of silicon semiconductor integrated circuits the dimensions of MIS (Metal Insulator Semiconductor) semiconductor devices have also decreased. According to ITRS (International Technology Roadmap for Semiconductors; updated version in 2000), the 100-nm technology node needs a gate insulation film having an equivalent oxide thickness (hereinafter abbreviated EOT) of 1.0 to 1.5 nm. At this film thickness, an insulation film having a higher dielectric constant than those of silicon oxide film and lightly N-doped silicon oxynitride film is necessary to achieve a gate insulation film with suppressed leakage current. Examples of suitable gate insulation films having a high dielectric constant include heavily N-doped silicon oxynitride film, silicon nitride film, and high dielectric metal insulation films of Al-, Zr-, or Hf-based silicates (AlSiOx, ZrSiOx, and HfSiOx).
However, gate insulation films using high-dielectric-constant materials have several associated problems. For example, the nitrogen concentrations near the interfaces of silicon oxynitride film and silicon nitride film increase, thus increasing the interface state density and positive fixed charge density. Accordingly, the transistor's mobility deteriorates. Similarly, increases in the metal densities in Al-, Zr-, and Hf-based silicate films increase the negative fixed charge density near the interfaces, thus also deteriorating the transistor's mobility.
To address these problems, a method of inserting a silicon oxide film into the high-dielectric-constant insulation film/Si interface has been proposed. The insertion of the silicon oxide film attempts to reduce the interface state density at the insulation film/Si interface, thereby moving fixed charges away from the Si substrate and suppressing mobility deterioration. However, when the oxide film thickness exceeds 1 nm, the dielectric constant of the whole film inevitably decreases. Therefore, there remains a critical need for a method of forming an ultrathin (<1 nm), homogeneous (>8 inches), high-quality, silicon oxide film.
In an effort to satisfy this need, the following three procedures have been proposed to form an ultrathin silicon oxide film prior to the formation of a high-dielectric-constant insulation film.                Low-Temperature Oxidation (including radical processes)        Modification of chemical oxide formed by pretreatment        Rapid Thermal Oxidation (RTO)        
However, these proposed procedures also have problems.
With respect to the low-temperature oxidation procedure, the density of the formed film is low, which reduces the reliability against electrical stresses. In the radical oxidation procedure, the in-plane uniformity deteriorates under the influence of the distribution of oxygen radicals and variations in the oxygen radical lifetimes.
In the modification of chemical oxide procedure, the annealing step results in film thinning due to the low oxygen partial pressure utilized. Moreover, SiO desorption from the interface produces pinholes, and thus film nonuniformity.
With respect to the RTO, the initial oxidation rate is high and, therefore, it is difficult to control the film thickness in thin-film regions of less than 1 nm. Furthermore, SiO2/Si interfaces formed at a reduced pressure give rise to rough and nonuniform films. In this thermal oxidation procedure, SiO desorption (Si+SiO2→2SiO↑) occurs at the SiO2/Si interface. It is also theoretically expected that release of Si (or SiO gas) from a Si substrate to an oxide film occurs during initial oxidation of the Si surface (H. Kageshima et al., Jpn. J. Appl. Phys. 38, L971 (1999)). It is known that this Si (SiO) diffuses through the oxide film and becomes a source of interface state densities and fixed charges (Takakuwa et al., Formation, Characterization, and Reliability of Ultrathin Silicon Oxides (4th Workshop), JSAP Catalog No. AP992204, 99 (1999)).
It has been confirmed that during annealing of an oxide film even under a high vacuum, pinholes are formed in the oxide film, the interface is roughened, and the amount of sub-oxides in an incompletely oxidized state increases (J. V. Seiple et al., J. Vac. Sci. Technol. A13(3), 772 (1995) and N. Miyata et al., J. Appl. Phys. 74(8), 15, 5275 (1993)).
It can be seen from the foregoing that an optimum range of oxidizing species partial pressures and oxidation temperatures that suppresses both growth of silicon oxide film and SiO desorption is very narrow. This is a great impediment to application to a production line.
Therefore, there remains a critical need for a high throughput method of forming an ultrathin (<1 nm), homogeneous (>8 inches), high-quality, silicon oxide film, which is amenable to large scale production.